The field of molecular electronics has developed rapidly over the past 15 years with the discovery of organic conducting and semiconducting compounds. Over this period, a large number of compounds has been discovered, that have semi-conducting or electro-optic characteristics. It is generally understood, that molecular electronics will not displace conventional, silicon-based semiconductor building blocks. Instead, it is assumed that molecular electronic structural elements will open up new applications in which suitability for coating large areas, structural flexibility, processability at low temperatures and low costs are required. Semiconducting organic compounds are currently used for applications such as organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), sensors and photovoltaic elements. By simple structuring and integration of OFETs into integrated organic semiconductor switches, cost-effective solutions become possible for intelligent cards (smart cards) or price labels, which it has so far been impossible to realise using silicon technology because of the price and the inflexibility of silicon building blocks. Equally, OFETs could be used as switch elements in large-scale flexible matrix displays. A summary of organic semiconductors, integrated semiconductor switches and their use is given for example in Electronics 2002, Volume 15, p. 38.
A field effect transistor is a tri-electrode element, in which the conductivity of a thin conducting channel between two electrodes (called the ‘source’ and the ‘drain’) is controlled by a third electrode (called the ‘gate’), separated from the conducting channel by a thin insulating layer. The most important characteristic properties of a field effect transistor are the mobility of the charge carrier, which is decisive in determining the switching speed of the transistor and the ratio of the currents when switched on and when switched off, known as the ‘on/off ratio’.
Two major classes of compound have been used hitherto in organic field effect transistors. All of the compounds have long conjugated units and are sub-divided into conjugated polymers and conjugated oligomers, depending on their molecular weight and structure.
In general, oligomers have a uniform molecular structure and a molecular weight below 10000 Dalton. Polymers generally consist of chains of uniform repeating units with molecular weight distribution. However, there is a continuous transition between oligomers and polymers.
When distinguishing between oligomers and polymers, it is often said that there is a fundamental distinction in the processing of these compounds. Oligomers are often evaporable and are applied to substrates by vapour deposition processes. Compounds that are no longer evaporable and must thus be applied by a different process are often described as polymers irrespective of their molecular structure. With polymers, the aim is generally to produce compounds that are soluble in a liquid medium, for example organic solvents, and can then be applied by corresponding application processes. A very common application process is e.g. ‘spin-coating’. The application of semiconducting compounds by the Ink-Jet process is a particularly elegant method. In this process, a solution of the semiconducting compound is applied to the substrate in the form of very fine droplets and dried. This process allows structuring to be carried out during application. A description of this application process for semiconducting compounds is given for example in Nature, Volume 401, p. 685.
In general, greater potential is attributed to the wet chemical process for obtaining low-cost organic integrated semiconductor switches by simple means.
An important prerequisite for the production of high-quality organic semiconductor circuits is compounds of extremely high purity. Ordering phenomena play an important role in semiconductors. The inhibition of uniform alignment of the compounds and marking of grain boundaries leads to a dramatic fall in semiconductor properties, so that organic semiconductor circuits built using compounds that are not of extremely high purity are generally unusable. Remaining impurities may, for example, inject charges into the semiconducting compounds (‘doping’) and thus reduce the on/off ratio or act as charge traps and thus drastically reduce mobility. Furthermore, impurities may initiate a reaction of the semiconducting compounds with oxygen and impurities with an oxidising action may oxidise the semiconducting compounds and thus reduce possible storage, processing and operating times.
The purities generally needed are so high, that they generally cannot be achieved by the known processes of polymer chemistry such as washing, reprecipitation and extraction.
Oligomers, on the other hand, which are often volatile compounds of a uniform molecular structure, can be purified relatively simply by sublimation or chromatography.
Some important examples of semiconducting polymers are described below. For polyfluorenes and fluorene copolymers, for example poly(9,9-dioctylfluorene-co-bithiophene) (I)
charge mobilities, abbreviated below also to mobilities, up to 0.02 cm2/Vs were achieved (Science, 2000, Volume 290, p. 2123), with regioregular poly(3-hexylthiophene-2,5-diyl) (II)
even mobilities up to 0.1 cm2/Vs (Science 1998, Volume 280, p. 1741). Like almost all other long-chain polymers, polyfluorene, polyfluorene copolymers and poly(3-hexylthiophene-2,5-diyl) form good films after application from solution and are therefore easy to process. However, as high-molecular weight polymers with molecular weight distribution, they cannot be purified by vacuum sublimation and are difficult to purify by chromatography.
Important representatives of oligomeric semiconducting compounds are, for example, oligothiophenes, in particular those with terminal alkyl substituents according to formula (III)
and pentacene (IV)

Typical mobilities for e.g. α,α′-dihexylquarter-, -quinque- and -sexithiophene are 0.05-0.1 cm2/Vs.
Mesophases, in particular liquid crystalline phases, appear to play a particular role in semiconducting organic compounds, which the persons skilled in the art have so far not completely understood. For example, the maximum mobility has so far been reported for crystals of α,α′-dihexylquarterthiophenes (Chem. Mater., 1998, Volume 10, p. 457), which crystallise at a temperature of 80° C. from an enantiotropic liquid crystalline phase (Synth. Met., 1999, Volume 101, p. 544). Particularly high mobilities can be obtained by using individual crystals, e.g. a mobility of 1.1 cm2/Vs was disclosed for individual crystals of α,α′-sexithiophene (Science, 2000, Volume 290, p. 963). If oligomers are applied from solution, there is mostly a sharp drop in mobility.
In general, the drop in semiconducting properties during processing of oligomeric compounds from solution is caused by the moderate solubility and low film-forming tendency of the oligomeric compounds. Thus inhomogeneities are caused for example by precipitation during drying from the solution (Chem. Mater., 1998, Volume 10, p. 633).
Attempts have therefore been made to combine the good processing and film-forming properties of semiconducting polymers with the properties of semiconducting oligomers. Patent specification U.S. Pat. No. 6,025,462 discloses conductive polymers with a star structure, which consist of a branched core and a shell of conjugated side groups. However, these have some disadvantages. If the side groups are formed from laterally unsubstituted conjugated structures, the resulting compounds are insoluble, or not readily soluble, and cannot be processed. If the conjugated units are substituted with side groups, although this leads to improved solubility, the steric requirements of the side groups cause internal disorientation and morphological defects, which impair the semiconducting properties of these compounds.
The laid open specification WO 02/26859 A1 discloses polymers of a conjugated reverse wheel to which aromatically conjugated chains are attached. The polymers bear diarylamine side groups, which allow electron conduction. However, as these compounds are based on diarylamine side groups, they are unsuitable as semiconductors.
The laid open specification EP-A 1 398 341 discloses semiconducting dendrimers, which can be processed from solution.
However, further improved compounds are needed, which combine the semiconducting properties of known oligomers with the processability and good film-forming properties of known polymers.